I'm on the Fallout 2 quest in NCR to kill Vortis and free the slaves, I killed the slavers, but I can't free the slaves. Whenever I click on the terminal, it says "invalid code sequence." I've been stuck here for ten minutes, just clicking the terminal. Is this a bug?

are you just clicking on it, or are you using relevant skills on it?
How high are the relevant skills?

No. It means you failed in your attempt to hack the terminal. Its a bit like shooting a gun, its not guaranteed.

You can keep retrying (it is a semi-random chance), improve your science skill and come back later, or try an alternative method (If I recall correctly the doors can be lockpicked directly one by one)

I click on it, or I used science (only 15) and it's the same result.

15 is far too low. Bear in mind that NCR is "late game", and to succeed on any skill check in a late game area you need a high skill level.

your science skill of 15/300 isnt good enough to hack the terminal.